Spread a side of each bread slice with butter. Turn 6 slices over; top with cheddar, then tomatillos, bacon, cilantro, and panela. Top with remaining bread, buttered side up.
Cook sandwiches in a large nonstick frying pan (work in batches or use 2 pans) over medium-low heat until golden on both sides and cheese melts, 7 minutes total.
*Panela, a fresh Mexican cheese, melts without losing its shape. Find it at Mexican markets.